The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for driving a bipolar junction transistor. Merely by way of example, the invention has been applied to drive a bipolar junction transistor using a base current that changes with time. But it would be recognized that the invention has a much broader range of applicability.
Bipolar junction transistors (BJTs) have been widely used as power switches in power electronic systems. FIG. 1(A) shows a simplified cross-section of a conventional N-P-N bipolar junction transistor (BJT). The N-P-N BJT 102 includes a p-type-doped layer 106, a lightly n-type-doped layer 108, and a heavily n-type-doped layer 110. The layer 110 is connected to a terminal 118 (e.g., the terminal “C” representing a collector), and the layer 108 serves as a collector drift region. As shown in FIG. 1(A), there are three heavily-doped regions in the layer 106, including a heavily p-type-doped region 112 and two heavily n-type-doped regions 114. The region 112 is connected to a terminal 116 (e.g., the terminal “B” representing a base), and the regions 114 are connected to a terminal 120 (e.g., the terminal “E” representing an emitter). Often, the N-P-N BJT 102 is turned on by injecting a current into the region 112 which causes electrons to flow from the regions 114 to the layer 110. FIG. 1(B) shows a simplified schematic symbol for the conventional N-P-N bipolar junction transistor 102. The schematic symbol 104 includes the terminals 116, 118, and 120, which represent the base, the collector, and the emitter, respectively. An arrow 122 for the terminal 120 points in the direction of a current flow if the N-P-N BJT 102 is turned on.
FIG. 2 is a simplified conventional diagram showing the collector current as a function of collector-to-emitter voltage of the N-P-N BJT 102. As shown, the N-P-N BJT can operate in at least a linear region, a quasi-saturation region, and a hard saturation region. In the linear region, the collector current (e.g., Ic) remains constant with respect to the collector-to-emitter voltage (e.g., Vce) for a specific base current (e.g., Ib). Additionally, if the collector-to-emitter voltage (Vce) is sufficiently reduced, the BJT 102 enters the quasi-saturation region, and if the collector-to-emitter voltage (Vce) is sufficiently further reduced, the BJT 102 enters the hard-saturation region.
The power switches used in power electronic systems often are required to provide a high switching speed, a low on-state output impedance, and a high off-state output impedance. Thus, as a power switch, the BJT 102 usually operates in the hard-saturation region when the BJT 102 is turned on in order to keep the output impedance low. But the maximum switching frequency of the BJT 102 often is limited in the hard-saturation region. For example, when the BJT 102 enters the hard-saturation region, a lot of minority carriers are accumulated in the base; therefore these minority carriers usually need to be removed before the BJT 102 can be turned off. The time needed for removing the accumulated minority carriers is referred to as a storage time, which represents the time when the BJT 102 remains on even after the base current has dropped to approximately zero. Therefore the storage time of the minority carriers can limit the maximum switching frequency of the BJT 102.
To raise the maximum switching frequency of the BJT 102, the amount of the minority carriers stored in the base need to be reduced. For example, a negative base current is used to sweep out the minority carriers from the base of the BJT 102. But, when the BJT 102 operates in the hard-saturation region, it often is difficult to quickly turn off the BJT 102 by using the negative base current, because before the BJT 102 is turned off, carriers would be stored into the base region of BJT 102.
In another example, in order to reduce the amount of the minority carriers stored in the base, the BJT 102 is prevented from entering the hard-saturation region so that the BJT 102 can be turned off quickly. But this approach can significantly increase the on-state power consumption of the BJT 102. The collector-to-emitter voltage (e.g., Vce) usually is higher in the quasi-saturation region than in the hard saturation region for the same base current in order to generate the same collector current.
FIG. 3(A) is a simplified diagram showing a conventional flyback power conversion system. The flyback power conversion system 300 includes at least a BJT 304, a controller 306, and a resistor 308. The BJT 304 (e.g., the BJT 102) is used as a power switch, and the controller 306 is used to drive the BJT 304. The BJT 304 includes an emitter, a collector, and a base, and the controller 306 includes terminals 310 and 312. The emitter of the BJT 304 is connected to the resistor 308, and the base of the BJT 304 is connected to the controller 306 through the terminal 310 (e.g., a terminal “DRV”). As shown in FIG. 3(A), the controller 306 provides a base current 305 through the terminal 310 in order to turn on or off the BJT 304. If the BJT 304 is turned on, an emitter current of the BJT 304 flows through the resistor 308, which generates a voltage signal 309. The voltage signal 309 is received by the controller 306 through the terminal 312 (e.g., a terminal “CS”).
FIG. 3(B) is a simplified conventional timing diagram for the flyback power conversion system 300. The waveform 314 represents turned-on and turned-off conditions of the BJT 304 as a function of time, the waveform 316 represents the base current 305 as a function of time, and the waveform 318 represents the voltage signal 309 as a function of time. As shown in FIG. 3(B), when the BJT 304 is turned on (e.g., during t0), the base current 305 remains constant, and the voltage signal 309 increases over time.
This conventional technique of driving the BJT 304 may turn on the BJT 304 and quickly drive the BJT 304 into hard saturation so as to reduce power consumption during the turn-on process. But the constant base current 305 (e.g., as shown by the waveform 316 during t0) often makes it more difficult to sweep out the minority carriers stored in the base of the BJT 304 during the turn-off process. Hence, the turn-off process for the BJT 304 often is long, and the power consumption of the BJT 304 can be high.
Hence it is highly desirable to improve techniques of driving a bipolar junction transistor.